An isotropic antenna transmits and/or receives power in all directions equally. Such an antenna is considered to have an isotropic pattern or directivity of 1 (0 dBi). An isotropic antenna has no preferred direction of radiation. If an antenna is non-isotropic, then the response will favor one or more directions over others. In the favored directions, the directivity will be more than 1 (>0 dBi), and in the non-favored directions, the directivity will be less than 1 (<0 dBi).
A term that is useful in defining antenna performance is gain. The gain (G) of an antenna is determined by its directivity (D) multiplied by its efficiency:G=D×efficiency  (1)
As antenna size is reduced relative to operating wavelength (A), efficiency will typically decrease and thus gain will also decrease. This makes building small antennas with adequate gain a challenge. There are several techniques for increasing antenna gain. Some of the techniques include building a larger antenna and/or building an antenna inside a high dielectric material. The high dielectric material slows the speed of light around the antenna, effectively making the antenna perform as if it were larger.
Using multiple antennas (or elements) is another technique for increasing antenna gain. Two basic antenna configurations that include multiple elements are broadside and end-fire array antennas. In a broadside array, the elements are arranged on a plane and maximum directivity is along a direction normal to the plane. An example of a four-element broadside array is shown in FIG. 1. In an end-fire array, the elements are arranged axially and maximum directivity is along a direction parallel to the axis. An example of a four-element end-fire array is shown in FIG. 2. In both the broadside and end-fire arrays, all elements are driven (or connected to a radio). A parasitic array is another antenna configuration that includes multiple elements, but in a parasitic array, at least one element is driven and at least one element is not driven (or not connected to a radio).
Improved antenna designs and configurations are constantly sought to increase gain based on the specific requirements of particular applications.